With the advent of modern technology, the need has arisen for components and structures of uprecedented efficiency. Such structures require both a greatly improved understanding and exploitation of the engineering properties of classical materials plus the development and use of new materials, such as non-metallics and composites. Along with these developments, a need has arisen for commensurate improvements in the technology of holographic non-destructive testing (HNDT).
There presently exists a multitude of non-destructive testing techniques, each having its own applications, advantages and limitations. The primary limitation of each of these techniques is that they may only be used on certain test objects or to determine the presence of only certain flaws, debonds or inhomogeneities. Three of these techniques are the optical holography, acoustical holography and holographic correlation techniques. The optical holographic system is described in U.S. Pat. No. 3,883,215, issued to the present inventor, Robert Kurtz. This patent describes a holographic system for the sequential non-destructive testing utilizing a real time anaylsis method, a double exposure method and a time-averaging method all with the same optical system. This system can accommodate all forms, shapes, sizes and geometries of known objects which demand non-destructive testing. This then alleviated the need for many different arrangements of holographic systems, each one of which only accommodated individual materials and/or objects. However, because optical holography is dependent on a subsurface flaw or debond making its presence known by virtue of a concomitant change on the surface of the object, many of said flaws or debonds cannot be discovered by utilizing this technique alone.
Acoustical holography, on the other hand, has the advantage of utilizing long wavelength radiation and, consequently, materials normally opaque to the optical wavelengths are transparent to this wavelength region. Therefore, acoustical holography allows for the object to be inspected throughout its entire volume which would otherwise be impossible using optical holography. However, one of the prime disadvantages of acoustical holography is that of resolution in its detection and recording techniques. Therefore, to properly test certain objects, one would need to employ more than one technique and, consequently, one would then have to inspect more than one hologram and/or type of hologram of the same scene in order to obtain all of the positive information about the test objects.
The correlation technique has a very distinct advantage in flaw detection and has been employed repeatedly for such testing of printed circuit boards, etc. This technique basically operates on the principle of correlation of intensity returned from some small area of tests. The magnitude deviation of the returned intensity (of an optical beam) is a direct function of the location and magnitude of the surface change, i.e., or flaw. The disadvantage here is that it is restricted to operate over a relatively small area and, consequently, could not be well applied for inspection of large objects. Therefore, to properly test a large object, all three of these holographic techniques are necessary, and, consequently, a minimum of three separate holograms may have to be interpreted.